1. Field of the Invention
The present invention relates to a blue laser source which efficiently generates laser radiation having a wavelength near 460 nm, and in particular, relates to a blue laser source using Thulium doped ZBLAN fluoride glass inside the cavity of a Nd:YAG gain medium.
2. Description of the Related Art
A laser generally comprises a gain medium producing a light beam and two mirrors defining a laser cavity around the gain medium to reflect a light beam back and forth through the gain medium. One of the mirrors, the exit mirror, is specially coated to allow a beam of light, the laser beam, having a predetermined wavelength to pass therethrough. The gain medium is pumped by a light source, such as a diode or another laser. Presently, the only lasers capable of emitting light in the blue spectral region are gas lasers such as an Argon ion laser, and a HeCd laser. However, these known blue gas lasers have a very low electrical to optical conversion efficiency, less than 0.01%. As such, these lasers require many kilowatts of electrical power to generate optical power in the watts range. Such high power requirements dictate that high powered blue gas lasers are only useful in limited laboratory applications. The usefulness of such blue gas lasers is also limited by the large amount of excess heat that has to be removed by the laser cavity. For example, known high-powered blue gas lasers typically have a cavity length in the range of one to two meters. When generating a laser above 100 mW of optical power, circulating water and external chillers are required. Air cooling, using a fan, has been successfully used for blue gas lasers outputting less than 100 mW of optical power, allowing use in applications such as high speed laser printing, IC wafer alignment, IC mask generators, and CD mastering systems. However, the known powered air cooled blue gas lasers still entail a relatively large cost and have a low efficiency. Further, due in part to the mechanical vibrations caused by the fan, known low powered air-cooled blue gas lasers have a relatively short lifetime of approximately 5000 hours. This limits the usefulness of the low powered air cooled blue gas lasers in commercial applications.
Several solid state alternatives to blue gas lasers have been proposed. These include: fiber upconversion lasers, directly frequency doubled laser diodes, sum frequency mixing between a Nd:YAG laser and a laser diode, and sum frequency mixing of laser diode emission with Nd:YAG power in an intracavity mounted nonlinear crystal. However, each of these systems has a maximum output power of 100 mW, due to the use of diffraction limited laser diode pumps which are limited to less than one watt of power. While the fiber upconversion laser can be pumped by a higher power Nd:YAG laser, its output power is limited by the high density in the small single mode fiber core which undergoes catastrophic fiber damage when the pump power exceeds a few watts.